For example, as shown in FIG. 5, an impeller 10 for an centrifugal compressor is configured of a hub 11 with one surface fixed to a rotation main axis of the centrifugal compressor for rotation being curved to be thinner toward its tip, a shroud 12 in the shape of facing the curved surface of the hub 11, and a plurality of blades 13 provided so as to partition a curved surface between the hub 11 and the shroud 12 in a swirl shape. Among the hub 11, the shroud 12, and the blades 13, a flow path 14 for compressed gas is formed.
Examples of the impeller 10 include one called a three-piece type in which the hub 11, the shroud 12, and the blades 13 are individually produced and joined to one another and one called a two-piece type in which the shroud 12 and the blades 13 (or the hub 11 and the blades 13) are integrally fabricated and are joined to the hub (or the shroud 12) fabricated separately therefrom. In the impeller 10 of any of these three-piece type and two-piece type, joining is made by welding or brazing. Whether joining is made by welding or brazing is determined based on the size, strength, and others of the impeller 10.